The placebo effect is a wonderful thing. Inert treatments provoke real medical benefits simply by virtue of the patient expecting the intervention to help. But of course, a lot of times placebos don’t work, and some people seem to be more responsive to their benefits than others. A new study by researchers at the University of Michigan has set about discovering if personality plays a role here, specifically in relation to placebo treatment for pain.

Marta Peciña and her colleagues provoked pain in 47 men and women by injecting hypertonic saline solution into their jaw muscle. Pain levels without treatment were then compared against the participants’ experience each time they received a placebo, in the form of a 15 second intravenous delivery of harmless isotonic saline solution. All the while, the participants were scanned via PET, to see how much activity occurred at the brain’s opioid receptors. This was to provide an objective measure of the activity of the brain’s own pain relief system.

There was a clear relationship between participants’ scores on various personality measures and their responsiveness to placebo. A mix of ego resilience (measured by statements like “I quickly get over and recover from being startled”); high agreeableness (especially altruism and honesty); and low neuroticism (especially low levels of angry hostility) accounted for 25 per cent of the variance in participants’ degree of response to the placebo treatment. Moreover, the participants who matched this pattern of traits tended to show more opioid receptor activation in their brains. Surprisingly perhaps, placebo responsiveness was not related to a person’s general optimism.

Why should a person’s agreeableness be related to their response to placebo treatments? “In the patient-doctor relationship, agreeableness appears likely to contribute to a strong therapeutic alliance,” the researchers said, “as well as to frank, collaborative feedback through the therapeutic process. Thus, it appears that individuals high upon this trait are particularly well equipped to fully engage in therapeutic efforts, and in this sense, be a good responder to treatment, even if it is placebo.” Meanwhile, the finding for angry hostility fits with past research showing that angry people tend to exhibit less indigenous opioid activity in their brains.

Peciña and her team said their findings, if replicated, could help with future pain research. “Simple to administer measures may aid in the interpretation of clinical trials and the stratification of clinical research volunteers to reduce variability in therapeutic responses,” they said.